Submarine signaling



pee. 27, 1927.

J. J. GILBERT SUBMARINE SIGNALING Filed A112. 28. 1924 Patented Dec. 27, 1927.

"PATENT ewes;

UNITED STATES JOHN J. GILBERT, OF DO'UG-LASTOET, lilEl/V YUEK,"ASE%IGIE\TOE TO ELECClRIC COMPANY, INCORPORATED, OF NEW YQPL-ii, N. A CORPORATION OF NEW YORK.

SUBMARINE fi IGNALlNG.

This invention relates to submarine telegraph systems and more particularly to receiving equipment for such a system.

1n submarine telegraph systems t 1s custon'iary practice to connect the receiving apparatus between the submarine cableand a local ground or between the cable and a low resista ce conductor leading to'a ground at sea. By using suitable receiving equipment a maximun'i signal response'tor such an arrangement is obtained. For duplex opera tion however, an artificial [cable so constructed as to simulate the electrical characteristics oi the'real cable, is connected between the receiving apparatus and the ground connection. in such system, the signal response is materially reduced due to the added impedance of the artilicial cable. Another example or sacrificing signal response in order to obtain certain desirable results is the use of a cable balancing return circuit to reduce interference local to the shore end. i I 7 y I The cable balancing return circuit comprises a conductor one terminal 01 which is located in proximity to oneterminal of the main cable and the other terminal otthe conductor is connected to a ground at sea through a suitable network, the impedance of the conductor, network and ground as a whole substantially balancing the impedance of the main cable. The conductor of the cable balancing return circuit may preferably be located throughout its whole length in proximity to the main cable in order that both may beequally' affected by any outside source of electrical disturbance. Such a system is described in British Patent 218,261, Aug. 31, 1925.

An object of the present invention is to improve the signal response when a network having substantial impedancev is connected between the and ground. I c

rtnother ob ect is to improve the signal response in a submarine telegraph system receiving" equipment employing an artificial cableor a, cable balancing return circuit. 1 I a In systems heretofore knownthereceiving apparatushas had an input impedance for the frequency components essentijal to signaling of about the same magnitude as'the characteristic.impedance of the cable or or lower magnitude. According. to the present invention the input impedance of the receivmg apparatus in duplex systems employing an artificial cable or in SlHlPlQX systems employing a cable balancing return circuit, is

made as high as possible in order that the signal response may be substantially the same as would be the case 11' a local ground were used. in a spccilic embodiment of the invention a three electrode electron'discharge amplifier having a very high input impedance is used in connection withjthe high terminating impedance.

the novel features which are considered characteristic this invention are set forth circuit terminated in a high impedance ac cording to this invention.

.Rer'erring now to the drawing a single, cor submarine cable 5 is connected to 'receiving equipment it by means of a twin core'cable section 6 ancing return circuit. between the lines and YY constitutes the twin core-section including the balancing network and groundat sea. One of the twin cores 7 is connected to the core 8 of the single core cable at the line XX while the other end is connected to the receiving apparatus B through switch 9. i The other core 10 oi the twin core cable 6 is connected through balancing resistance 11 to the armor \vires'12 andthus to the sea water or earth at point 13. A conventional earth connection is shown in dotted lines to indicate that the resistance 11 which is connected to'the including a cable baltelegraph The section of cable armor wires shown by light lines 12 is ment 15. The output circuit of the amplifier A is connected to any suitable recording device 16, such for example, as a siphon recorder. A condenser 20 of relatively low impedance to those frequency components essential to signaling is connected between conductor 7 and the high impedance 15 in order to prevent very low frequency earth potential variations from affecting the am plifier A. This condenser may be of the order of 50 microfarads.

A suitable arrangement for amplifier A is that shown in Fig. 6 of patent to A. M. Curtis, 1,586,970, June 1, 1926.

For transmitting purposes a transmitter T may be connected between a local ground. 17 and the cable core 7 through a condenser 18 .by throwing switch 9 from the position shown to switch point 19. The signalin impulses are thus transmitted by way or the local ground instead of the ground at sea through core 10 and resistance 11. The same type of transmitting equipment is pro vided. at the distant end of the cable for transmittingpurposes and therefore has not been shown.

Signaling takes place in the well known manner. Code impulses are impressed upon the cable at the sending end and the received impulses are amplified in amplifier A and shaped by suitable networks before being re corded by recording device 16.

As is pointed out in applicants copending application, Serial No. 18,690, supra, the advantages of the use of the cable balancing return circuit are due to the fact that the interfering currents are reduced to amuch greater degree than the signal response. By the arrangement of the present invention the signal response is substantial ly unchanged by the introduction of the ground at sea with a resulting greater advantage in favor of the latter. This advantage may be expressed by saying that the high impedance termination for frequency components essential to signaling makes possible a more favorable ratio of signal strength to out of balance interference. I

An explanation for the improved results due to the high impedance termination of the cable and balancing return circuit can probably best be given. mathematically. Such an explanation will now be given.

Let 7, 7c and a be respectively the propagation constant, characteristic impedance and length of the single core cable; and la and Z) be the corresponding quantities for the twin core between the ground at sea and the terminal. The impedance of the submerged artificial line will then be 70.

Let V and L, be the voltage and current impressed at the distant terminal and let V and I be the voltage and current at the receiving apparatus, which has an imped- We can assume for the present purpose /0=27c and =-y.'

Three special cases are of particular interest in showing what effect the value of.

Z has upon the efficiency of reception.

Case 1.Z =27:.

That is, the terminal impedance is made equal to the characteristic impedance of the twin core cable. This is a condition that gives maximum energy of the received signal and is the condition that is ordinarily approximated in submarine. cable operation. Placing this value of E in the above formulae we get and I The received voltage is thus exactly the same as if the ground at sea were absent and the main cable were terminated in a network having an impedance equal to the characteristic impedance Ila.

value that would be obtained if. the cable balancing return circuit were not employed;

thus the effect of the return circuit in this case is to reduce the maximum energy that can be received to one-half the value that can 1be received where the sea earth is not usec.

Case t. Z O.

This represents the case where the terminal impedance is made very small compared with the characteristic impedance of the cable, which arrangement would be "employed if it were desired to obtain large currents from the cable at the expense of the voltage. From formula (1) we see that in this case I 'lhecurrent is thus only one-half the value that would be obtained by use of a low im- The current, however, accordingv to (2) 1s only half the.

This represents the case where the impedance of the receiving apparatus is very large compared with the characteristic impedance,

of the twin core cable. Vi e have for this case V 2V e and balancing return circuit it is desirable tomake the impedance of the receiving network large compared with the characteristic.

impedance, or more generally, the input impedance of the twin core cable for the tre quencies ifor which as large an amplitude as possible is desirable. These frequencies will lie in the range between the signaling frequency and a value about 1.5 times the signaling frequency.

From the above explanation it is evident that best results would be obtained if the terminal impedance could be made'infinite. This is physically impossible in anymaterial installation but this ideal relationship can be approximated. In one embodiment of the invention the impedance ot the receiving equipment to the frequency components essential to signaling has been made as high as 3,000 to 6,000 ohms, as compared with 800 ohm-s for the characteristic impedcuit.

ance of the cable with balancing return cir- The reduction in signal response due to the return circuit was practically negligible. From this it follows that the imped ance oi": the terminal apparatus should be four times or morethe-magnitude of the characteristic impedance of the cable and balanced sea earth.

It is to be understood that embodiments of the invention other than the one illustrated in the drawing are possible without departing from the spirit of the invention as defined in the appended claims.

The expression long submarine cable is employed in the claims to indicate a submarine cable ot such length that in the major portion of the cable the magnitude ot the signaling current is substantially independent of the magnitude of the receiver im pedance facing the cable.

:Vhat is claimed is:

1. In a submarine telegraph system, a long submarine cable, a. cable balancing sea earth return circuit having a terminating impedance at its sea end approximately equal to the characteristic impedance of said cable, said return circuit having an impedance substantially equal to the input impedance of said submarine cable, and a receiver the in put impedance of which is at least three times the input impedance of said cable and return circuit combined.

2. In a submarine telegraph system, a long submarine cable having a characteristic impedance of approximately 400 ohms, a cable balancing sea earth return circuit having an impedance substantially equal to the characteristic impedance of said submarine cable, and a receiver theinput impedance of Which is atleast 8,000 ohms.

In witness whereof, I hereunto subscribe my name this 26th day of August A. D.,

JOHN J. GILBERT. 

